Move associated records of security policy bots to ghost user before deletion

When a security policy project is unlinked, we want to delete the security policy bot user from the project. The deletion of this user will be set to a pending state in case the user has associated records like vulnerability records.

To avoid this, we can move the associated records to a ghost user. For that, we have to change the worker and add a data migration to move associated records of security policy bots already pending for deletion.

Related comment: #444241 (comment 2025490104)